Fix check for _mouseSelection flag in dialogs.

Fixes #3781.

Telegram/SourceFiles/dialogs/dialogs_inner_widget.h

@@ -223,6 +223,7 @@ private:
 	DialogsList _contacts;
 
 	bool _mouseSelection = false;
+	QPoint _mouseLastGlobalPosition;
 	Qt::MouseButton _pressButton = Qt::LeftButton;
 
 	std::unique_ptr<ImportantSwitch> _importantSwitch;